This invention relates generally to electronic measurement instruments and in particular to a front-end architecture for an electronic measurement instrument using digital filters for extracting measurement parameters from digital samples.
Digital multimeters (DMMS) are a type of electronic measurement instrument which provide the ability to measure various physical parameters such as a.c. voltage and current, d.c. voltage and current, and resistance. Additional measurement capabilities are being added to new DMM designs, including diode check, capacitance, temperature, and frequency counter/timer measurements. Other more specialized measurement parameters may also be added for particular applications, such as measuring d.c. current in the microamp range for heating, ventilation, and air conditioning (HVAC) service applications.
Measuring a wider variety of measurement parameters has required the use of relatively complex signal conditioning circuits to receive the signal developed across the instrument input terminals and produce an input signal suitable for conversion to digital samples. Signal conditioning circuits may include ranging circuits consisting of analog amplifiers and attenuators to scale the input signal to a usable amplitude for the analog-to-digital converter (ADC). Signal conditioning circuits may also include voltage protection circuitry, such as mechanical relays, metal oxide varistors (MOVs), and positive temperature coefficient resistors, to prevent damage to the DMM when probing out-of-range voltages. Physical parameters such as a.c. and d.c. current flow, temperature, and pressure, must all be converted to an input voltage of suitable amplitude for conversion into digital samples by the ADC.
The input signal is split in the signal conditioning circuit of the DMM into two paths--an a.c. or high frequency path and a d.c. or low frequency path. The d.c. path of the input signal is typically developed by low pass filtering the input signal to ideally obtain only a d.c. voltage. The cut-off frequency of the actual low pass filter is typically less than ten hertz, allowing some a.c. signal content to be present. The d.c. voltage developed by the low pass filter is then provided to an ADC to produce digital samples. The low frequency path requires calibration for gain and offset voltage. The high frequency path requires calibration in terms of gain versus frequency to account for the low frequency roll-off.
The ADC provided in most DMMs has a maximum sample rate of less than 100 samples per second, but with 3 1/2 to 4 1/2 digits of resolution. In order to accurately measure a.c. signals, such as power line signals in the 50/60 hertz frequency range, a root mean square (rms) to d.c. converter is provided in the a.c. path to develop a d.c. voltage which is representative of the a.c. rms value. The rms converter is typically implemented in DMMs as a monolithic integrated circuit.
Digital storage oscilloscopes (DSOs) are another type of electronic measurement instrument that allows for digitally sampling the input signal for subsequent calculation of signal parameters. DSOs provide for switchable a.c. coupled and d.c. coupled paths for the input signal. ADC technology has evolved to provide sampling rates well over 100 megasamples per second, allowing the input signals to be converted directly to digital samples which are stored as a time record in acquisition memory. From this time record, the waveform and various signal parameters may be calculated. However, the signal conditioner in DSO front ends is optimized only for the acquisition of signal voltages. The ability to measure other parameters such as resistance or current, which are low frequency physical parameters, has not been incorporated into traditional DSOs. Hybrid measurement instruments have been developed that combine DMM technology for measuring a wide variety of physical parameters with DSO technology for waveform analysis.
In FIG. 1, there is shown a drawing (not to scale) of a measurement instrument 10 having a pair of test probes 12a and 12b for measuring a variety of physical parameters including a.c. volts and current, d.c. volts and current, resistance, and capacitance, among others. The measurement instrument 10 may also provide for testing passive two-terminal components such as diodes using current and voltage sources (not shown) to generate a stimulus signal across the component. It is desirable that the measurement instrument 10 have the versatility to measure a wide variety of physical parameters.
On a front panel of the measurement instrument 10, there is mounted a graphical display 14 which may show numerical measurement parameters such as "117 VAC rms" in the manner of a DMM as well as a graphical representation of the waveshape of the input signal in the manner of a DSO. The measurement instrument 10 may be coupled to a voltage source 16, a current source 18 shown in FIG. 2, or a component 20 shown in FIG. 3. The component 20 may comprise any of a variety of passive, two-terminal components, including resistors, capacitors, inductors, diodes, or any other two-terminal device amenable to measurement and analysis by the measurement instrument 10.
In FIG. 4, there is shown a simplified block diagram of a measurement front-end 98 according to the prior art as used in the Fluke 860 Graphical Multimeter measurement instrument. The pair of test probes 12a and 12b is coupled across the voltage source 16 to couple a voltage signal to a signal conditioner 50. The signal conditioner 50 may comprise amplifiers, dividers, and filters to provide an input signal of suitable amplitude and bandwidth for conversion into digital samples. The signal conditioner 50 may further comprise various forms of voltage protection circuitry (not shown) to prevent damage to the measurement instrument 10 from over-voltage and over-current conditions. The signal conditioner 50 may also comprise various circuits to convert various physical parameters into the input signal. For example, a.c. and d.c. current from the current source 18 are measured by developing a voltage drop developed across a calibrated current shunt or current clamp. Resistance is measured by measuring the voltage drop across the component 20 using a current source or voltage source (not shown) within the signal conditioner 50. Given the multitude of parameters that must be accommodated and converted to an input signal and the need to provide over-voltage and over-current protection, significant demands are placed on the signal conditioner 50, resulting in added circuit complexity and cost.
The input signal developed by the signal conditioner 50 is coupled to an input of a low pass filter 52 which produces a d.c. signal which is then coupled to a d.c. position of a switch 55. The low pass filter 52 typically has a roll-off frequency reasonably close to 0 Hertz to produce the d.c. component of the input signal while rejecting the a.c. components. The d.c. signal is supplied to a slow ADC 54 via the switch 55 in the d.c. position. The slow ADC 54 produces digital samples of the d.c. signal at a sample rate typically less than 100 Hertz but with 3 1/2 to 4 1/2 digits of resolution. The low pass filter 52 is typically used on input signals that are primarily d.c. in nature in order to produce accurate measurements of typical d.c. parameters such as d.c. voltage and current, and resistance.
An rms converter 56 also receives the input signal and is coupled to an a.c. rms position of the switch 55. The rms converter 56 is used for input signals that are a.c. in nature in order to produce a d.c. voltage that is representative of the rms value of the input signal which is supplied to the slow ADC 54 when the switch 55 is in the a.c. rms position. The rms converter 56, low pass filter 52 and slow ADC 54, in combination with the signal conditioner 50 collectively comprise the front end architecture commonly found in DMMs and is labeled the DMM FRONT END.
Additional waveform capability is provided in the measurement instrument 10 by adding circuitry collectively comprising a WAVEFORM FRONT END. A signal conditioner 51 accepts the input signal to produce a second input signal for conversion into digital samples. A fast ADC 58 receives the input signal from the signal conditioner 51 and produces digital samples at a sample rate substantially higher than the sample rate of the slow ADC 54 but typically at less resolution. An acquisition memory 60 receives the digital samples and stores them in an acquisition memory 60 to form a digital time record of the waveform of the input signal. A trigger 62 provides a trigger signal to determine the starting point of the waveform in the digital time record in the manner well known in the art for DSOs. A peak min/max 64 operates as a digital comparator to store the maximum and minimum values being stored in the acquisition memory 60 and provides the maximum and minimum values as digital samples.
The waveform front end comprised of the fast ADC 58, acquisition memory 60, and trigger 62 in conjunction with the signal conditioner 51 are typical of front end architectures found in DSOs. The input signal is acquired over a discrete acquisition time in order to fill the acquisition memory 60. A digital interface 66 receives the digital samples from the slow ADC 54, the peak min/max 64, and the acquisition memory 60 for use by the rest of the measurement instrument 10 as digital measurement values.
The signal conditioner 51 provides for voltage-protection and scaling similar to the signal conditioner 50 but in a manner that is optimized for waveform acquisition. The signal conditioner 51 allows for connection of the measurement instrument 10 to the voltage source 16 but not directly to the current source 18 or the component 20. Furthermore, different design considerations for the signal conditioner 51 from that of the signal conditioner 50, including frequency response flatness over a broader range of frequencies than that of the signal conditioner 50, may become important.
In FIG. 5, there is shown there is shown a simplified block diagram of a measurement front-end 99 according to the prior art as used in the Fluke 93, 95, and 97 oscilloscope measurement instruments. The pair of test probes 12a and 12b is coupled across the voltage source 16 to couple a voltage signal to a signal conditioner 70. The signal conditioner 70 may comprise amplifiers, dividers, and filters to provide an input signal of suitable amplitude and bandwidth for conversion into digital samples. The signal conditioner 70 may further comprise various forms of voltage protection circuitry (not shown) to prevent damage to the measurement instrument 10 from over-voltage and over-current conditions. Like the signal conditioner 51 shown in FIG. 4, the signal conditioner 70 is optimized for waveform acquisition.
A second pair of test probes 12a' and 12b' is coupled across the component 20 to allow for measurement of resistance or other component parameters. The pair of test probes 12a' and 12b' are coupled to a signal conditioner 70 which is optimized for low frequency measurements. In the preferred embodiment, there is no provision for the measurement of the current source 18. External current clamps or shunts may be used to provide a voltage signal to either of the signal conditioners 70 or 72. A switch 74 having a DIODE OHMS position coupled to the signal conditioner 70 and a VOLTS position coupled to the signal conditioner 72 selectively couples the input signal to an ADC 76 which digitizes the input signal and produces digital samples which are placed in acquisition memory 78. A trigger 80 which also receives the input signal and generates a trigger signal may be used to time the start of a particular data acquisition. The contents of the acquisition memory 78 may be analyzed to produce a variety of parameters which are provided to a digital interface 82 which in turn provides digital measurement values to the measurement instrument 10. While the measurement front end 99 requires only a single ADC 76, separate signal paths via the signal conditioners 70 and 72 and the switch 74 are maintained. Signal parameters are not extracted continuously but only from the portion of the input signal that is actually digitized, which may only be a small fraction of the total time.
The continued use of separate DMM and waveform signal paths, with separate signal conditioners optimized for waveform acquisition and low frequency DMM measurements, results in a substantial duplication of components and increased manufacturing cost and complexity in the measurement instrument 10. The ability of such measurement instruments to be adapted for measuring new types of signal parameters may also be severely limited because of this bifurcated structure. Therefore, it would be desirable to provide a front-end architecture for a measurement instrument that has only one path for the input signal that allows for the continuous extraction of multiple types of signal parameters.